Why Do Cumulene Ketones Kink?

We employ ab initio and density functional methods to investigate the equilibrium structure and vibrational frequencies of extended cumulene monoketones [CH2=(C=)(m)O] and diketones [O=(C=)(m)O], in order to elucidate the electronic origin of the curious kinked spine geometries that are common in such species. The dominant role of symmetry-breaking n(O)((pi))-sigma*cc interactions between the p-type lone pair of the terminal oxygen and adjacent unfilled CC antibonding orbital is demonstrated by NBO second-order delocalization energies, Fock matrix deletions, and natural resonance theory (NRT) descriptors, showing the general connection between cumulene kinking and CC bond-breaking reactions that split off CO. Our results provide simple rationalizations for (i) pronounced even/odd alternation patterns in the magnitude or direction of kinking, (ii) the nonexistence of O = C=C=O (iii) the clear preference for trans-like over cis-like kinks, and (iv) the extreme sensitivity of kinking with respect to weak perturbations, such as cage or solvent effects, remote chemical substituents, improved treatments of electron correlation, and the like.